A form of voltage arrester having a series circuit arrangement of voltage-dependent resistors and spark gaps as their primary component is known. It is further known to use voltage arresters of this type for protecting electrical operating equipment and installations for distributing power against surge voltages. The operation of these arresters is such that the spark gaps respond at a certain voltage level and that the energy contained in an over-voltage wave is discharged through the series circuit. In this manner, the surge voltages can have no damaging effect on the operating equipment or installations. Because of the voltage dependence of the resistor elements, the follow-on current caused by the system voltage is reduced to the extent that the spark gaps are able to interrupt it. The surge voltage arrester thereby returns to its non-conducting state.
It was possible in recent years to further improve the capacity of surge voltage arresters through the development of current-limiting spark gaps which supported the action of the voltage-dependent resistors. Spark gaps of this type are relatively costly since they require generally blow-out coils and complex quenching chambers which cause a lengthening of the electric arcs ignited between the electrodes.
Furthermore, surge voltage arresters for low operating voltages became known which include only voltage-dependent resistors and which therefore do not require spark gaps. In contrast to the surge voltage arresters of the type described first, these voltage-dependent resistors do not consist of silicon carbide but, rather, of a different semi-conductive resistance material based on zinc oxide doped with additional metal oxides. This resistor material exhibits a stronger voltage dependence than silicon carbide and therefore possesses a more pronounced valve action.
The application of surge voltage arresters utilizing metal-oxide resistors of the type mentioned is, however, limited to applications where a high ratio between residual voltage and extinguishing voltage is permissible. This circumstance exists in low-voltage networks where for instance the peak value of the extinguishing voltage amounts to 400 volts and where a residual voltage with a peak value of 2000 volts can be tolerated. Such a high ratio between residual voltage and extinguishing voltage is, however, already no longer permissible in medium voltage networks. For a peak value of the extinguishing voltage of 17 kV the here maximum permissible residual voltage amounts to 40 kV. The known metal-oxide resistors cannot be adapted to these conditions. A residual current flows during the peak value of the extinguishing voltage which would quickly destroy the resistor body.